Recent episodes involving the inadvertent consumption of methylmercury in food have caused death and severe illness in adults. Levels that cause no observable effects in pregnant women have caused severe neurological abnormalities in their infants. Using the monkey (Macaca mulatta, M. irus, and M. arctoides) because of its similarities to the human in pregnancy and fetal development, we have determined the minimum chronic intake levels during pregnancy that cause neurologic problems in the newborn. The extent of transplacental passage of methylmercury and phenylmercury has been measured as well as their localization in fetal tissue, with special attention to brain uptake and neuropathology. Mercury vapor, which can occur in dentists' offices and in factories, is being administered to pregnant macaques on a chronic basis and then measured in both mother and infant. The potential protective effects of penicillamine and polythiol resins against the toxic effects of methylmercury is being assessed in pregnant monkeys. Concern extends to pregnant women and their newborns for whom body burdens of mercury in this country need be established. Mercury levels in a rural population of women and their infants are low (l.0-1.2 ppb) although infant levels are nearly always higher than those of the mother because of greater affinity of the fetal red blood cells and perhaps other tissues for mercury. Currently, two urban populations are being assessed for mercury levels in mothers and offspring. Dietary and obstetric histories are being studied as possible determinants for the variations in body mercury burdens. Lastly, the rates of loss of mercury following birth in infants with the highest levels and in monkeys who were dosed with organic or inorganic mercury are being monitored.